1. Field of the Invention
The present invention relates to an epoxy resin composition suitable for use in, for example, sealing a semiconductor device and a cured product of the same.
2. Description of the Related Art
For packages of semiconductor devices wide use has been made of epoxy resin compositions which give a cured product having excellent electrical characteristics, mechanical characteristics, chemical resistance, moisture resistance, and other properties. Among these, resin compositions comprised of novolak type epoxy resins in which phenol resins are mixed as curing agents and an inorganic filler is added are currently the main stream for, for example, resin sealing of semiconductor devices.
In recent years, however, along with the increasing size of chips and their improved performance, the demands on the sealing materials have become severer, and now a sealing material is sought having both of a high heat conduction and a low stress characteristic.
In the past, in a semiconductor device with a power device sealed with a resin, the heat of the power device was radiated through the molding resin or the molding resin was used to directly protect the devices. Therefore, the heat conductivity and the moisture resistance of the molding resin had an effect on the reliability of the device as a whole. The molding resin was a mixture of an epoxy resin and filler. In general, the filler used was fused silica or crystalline silica, but epoxy resin compositions using crystalline silica end up with larger coefficients of heat expansion and it is difficult to obtain a low stress characteristic. In addition, to increase the heat radiated from the devices, it is necessary to use a filler with a high heat conduction. As such fillers, alumina, silicon nitride, and magnesia, are known, but nitrides (silicon nitrides etc.) and magnesia are not suitable for sealing semiconductors since they are broken down with water vapor and increase the rate of the deterioration of the resin. Alumina is sufficiently stable with respect to water vapor and is suitable as a semiconductor sealing material (for example, Japanese Unexamined Patent Publication (Kokai) No. 63-160254 and Japanese Unexamined Patent Publication (Kokai) No. 62-24031).
On the other hand, to improve the moisture resistance, it is important to reduce the impurity ions in the resin, in particular chlorine ions which promote corrosion of aluminum. Chlorine is coupled in the resin as a subreaction when the epoxy group is added to the resin at the stage of synthesis of the epoxy resin. This reacts with the catalyst added to increase the curing speed of the resin at the time of molding of the resin and is present as free chlorine ions. Therefore, it is necessary to select the type of the curing catalyst to reduce the chlorine ions. In particular, an organic phosphorous catalyst (triphenylphosphine (TPP) etc.) can reduce the chlorine more than an imidazole catalyst (for example, Japanese Unexamined Patent Publication (Kokai) No. 62-223218).
Thus, when using alumina as an inorganic filler, it is possible to obtain an epoxy resin composition which satisfies both the requirements of a high heat conduction and a low stress characteristic. On the other hand, when using an organic phosphorus compound as a curing accelerator, it is known, an epoxy resin composition can be obtained which has superior curing characteristics, storage, heat resistance, electrical characteristics, moisture resistance, etc. Therefore, in a semiconductor device where the power device is sealed by a resin, to simultaneously achieve a satisfactory heat dispersion and moisture resistance, it is necessary to use alumina as the filler and an organic phosphorus catalyst as the catalyst.
However, the following was learned from experiments by the present inventors. That is, Comparative Embodiment 1 in Table 1 concerned a combination of an alumina filler and an organic phosphorous catalyst (TPP). As shown in FIG. 4, when the resin was stored after manufacture (after mixing of the resin and filler) and this was used for sealing the power device, the resin's degree of cure fell along with the storage time, in particular, suffering from a remarkable drop in the curing characteristics when the composition absorbs moisture.
TABLE 1 __________________________________________________________________________ Comp. Embo. 1 Comp. Embo 2 Comp. Embo. 3 __________________________________________________________________________ Composition Resin (35 vol %) Epoxy resin + phenol resin Curing catalyst TPP.sup.Note 1 Imidazole TPP.sup.Note 1 Filler (65 vol %) Alumina Alumina Crystalline silica Curing characteristics (hot 55 89 89 hardness 10 days after manufacture) Moisture resistance 8 35 8 (impurity ion C1.sup.- ppm) Heat dispersion (heat 130 130 60 conductivity .times. 10.sup.-4 cal/cm.degree.C.s) Results of evaluation of product Heat dispersion (thermal 2 2 5 resistance .degree.C./W) Moisture resistance 600 200 3000 or more (pressure cooker test hours) __________________________________________________________________________ .sup.Note 1 TPP stands for triphenylphosphine (organic phosphorus catalyst)
Further, the present inventor previously, in Japanese Examined Patent Publication (Kokoku) No. 63-25010, proposed an epoxy resin composition containing an epoxy resin, a phenol resin as a curing agent, a curing accelerator, and an inorganic filler, characterized in that as the epoxy resin use was made of a cresol novolak epoxy resin having a content of organic acids of 100 ppm or less, a content of chlorine ions of 2 ppm or less, a content of hydrolyzing chorine of 500 ppm or less, and an epoxy equivalent of 180 to 230, as the phenol resin use was made of a novolak type phenol resin having a softening point of 80 to 120.degree. C., a content of organic acids of 100 ppm or less, free Na and Cl of 2 ppm or less, and free phenol of 1 percent or less, the molar ratio (a/b) of the epoxy groups (a) of the epoxy resin and the phenolic hydroxyl groups (b) of the phenol resin was adjusted to the range of 0.8 to 1.5, and, as the curing accelerator, use was made of a tertiary organic phosphine compound having in the molecule at least one functional group selected from a carboxyl group, methylol group, alkoxyl group, and hydroxyl group, more specifically 0.4 to 5 parts by weight of the compound shown by the following formula (I) per 100 parts by weight of the total of the epoxy resin and the phenol resin. ##STR1## wherein, R.sup.1 to R.sup.5 independently represent a hydrogen atom, carboxyl group, methylol group, alkoxyl group, and hydroxyl group and at least one of R.sup.1 to R.sup.5 is selected from a carboxyl group, methylol group, alkoxyl group, and hydroxyl group, and n is an integer of from 1 to 3.
The epoxy resin composition has superior curing characteristics, electrical characteristics at a high temperature, heat resistance (i.e., high glass transition temperature Tg), and moisture resistance and as a result prevents aluminum wire from corroding or breaking even when a device is left under high temperature and humidity conditions for a long period and further is superior in moldability and has a high long term storage stability.
The present inventor, however, engaged in further studies on the epoxy resin composition proposed in Japanese Examined Patent Publication (Kokoku) No. 63-25010 and as a result discovered that when an IC formed using the epoxy resin composition is allowed to stand under high temperature and humidity conditions in a state with voltage applied to it, the aluminum wiring frequently breaks. Therefore, it was desired to resolve this problem.